In my description of the mines of Santo Domingo I have only briefly alluded to the scarcity of alluvial gold in the valleys. It may be correlated with a similar scarcity in the glaciated valleys of Nova Scotia and North Wales, in the neighbourhood of auriferous quartz veins, and is probably due to the same cause. Glacier ice scoops out all the contents of the valleys, and in deepening them does not sort the materials like running water or the action of the waves upon the sea coast. I have in another place* (* "The Glacial Period in North America" by Thomas Belt. Published in "Transactions of the Nova Scotian Institute of Natural Science" 1866 page 91.) shown that in Nova Scotia, in the neighbourhood of rich auriferous quartz veins that have been greatly denuded, grain gold is only sparingly disseminated throughout the drifts of the valleys, whilst in Australia every auriferous quartz vein has been the source of an alluvial deposit of grain gold, produced by the denudation and sorting action of running water. When the denuding agent was water, the rocks were worn away, and the heavier gold left behind at the bottom of the alluvial deposits; but when the denuding agent was glacier ice the stony masses and their metallic contents were carried away, or mingled together in the unassorted moraines. That the transportation of boulders in Nicaragua was due to glaciers, and not to floating icebergs, may be argued on zoological grounds. The transported boulders, near Ocotal, are about three thousand feet above the sea, those near Libertad about two thousand feet. The low pass between the Atlantic and the Pacific oceans, through the valley of the San Juan and the Lake of Nicaragua, is less than two hundred feet above the sea,* (* See ante, Chapter 4.) and to allow for the flotation of icebergs at the lower of the two places named, a channel of more than eighteen hundred feet in depth would have connected the two oceans. This supposition is negatived by the fact that the mollusca on the two coasts, separated by the narrow Isthmus of Darien, are almost entirely distinct, whilst we know that since the glacial period there has been little change in the molluscan fauna, nearly, if not all, the shells found in glacial deposits still existing in neighbouring seas. In the Caribbean province, which includes the Gulf of Mexico, the West Indian Islands, and the eastern coast of South America as far as Rio de Janeiro, the number of marine shells is estimated by Professor C.B. Adams at not less than 1500 species. From the Panamic province, which, on the western coast of America, extends from the Gulf of California to Payta in Peru, there has been catalogued 1341 distinct species of marine molluscs. Out of this immense number of species, less than fifty occur on both sides of the narrow Isthmus of Darien. So remarkably distinct are the two marine faunas, that most zoologists consider that there has been no